In recent years, CO2 emission regulation has become tighter due to increasing environmental concerns. In the field of automobiles, weight reduction of car bodies has emerged as a challenge in improving fuel efficiency. Under such trends, automobile parts have become increasingly thinner by the use of high-strength steel sheets. In particular, high-strength cold-rolled steel sheets having a tensile strength (TS) of 1180 MPa or higher are now being increasingly used.
High-strength steel sheets used in structural parts and reinforcing parts of automobiles are required to have excellent formability. In particular, high-strength steel sheets used in parts having complicated shapes are required to excel in not only one but both of elongation and stretch flangeability (hereinafter may also be referred to as hole expandability). Moreover, automobile parts such as structural parts and reinforcing parts are required to have excellent impact energy absorbing properties. It is effective to improve the yield ratio of the steel sheets used as a raw material to improve the impact energy absorbing property of automobile parts. Automobile parts that use high-yield-ratio steel sheets can efficiently absorb impact energy despite low strain. The yield ratio (YR) is the value of the ratio of the yield stress (YS) to the tensile strength (TS) and is represented by YR=YS/TS. A steel sheet having a TS of 1180 MPa or more may undergo delayed fracture (hydrogen embrittlement) due to hydrogen from the working environment. Thus, to use a high-strength steel sheet having a TS of 1180 MPa or more, high press formability and excellent delayed fracture resistance are required.
Dual phase steel (DP steel) having a ferrite-martensite microstructure are known as high-strength steel sheets having both formability and high strength. For example, Japanese Unexamined Patent Application Publication No. 2011-052295 discloses a high-strength cold-rolled steel sheet having a good balance between elongation and stretch flangeability. This steel sheet has a particular composition and a microstructure containing an area ratio of 70% or more (including 100%) of tempered martensite having a hardness of higher than 380 Hv and 450 Hv or less and ferrite constituting the rest of the microstructure. The distribution of cementite grains in the tempered martensite is such that 20 or more cementite grains having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are present per square micrometer of the tempered martensite, and 1.5 or fewer cementite grains having an equivalent circle diameter of 0.1 μm or more are present per square micrometer of the tempered martensite. Japanese Unexamined Patent Application Publication No. 2011-052295 describes that when the hardness and area ratio of the tempered martensite and the cementite grain distribution in the tempered martensite are appropriately controlled in the dual phase microstructure composed of ferrite and tempered martensite, the balance between the elongation and stretch flangeability can be maintained while improving tensile strength.
Japanese Unexamined Patent Application Publication No. 2010-018862 discloses a high-strength cold-rolled steel sheet as a steel sheet having excellent workability and delayed fracture resistance. This steel sheet has a particular composition containing V: 0.001 to 1.00% and has a microstructure containing tempered martensite in an area ratio of 50% or more (including 100%) and ferrite constituting the rest of the microstructure. The distribution of precipitates in the tempered martensite is such that 20 or more precipitates having an equivalent circle diameter of 1 to 10 nm are present per square micrometer of the tempered martensite, and 10 or fewer V-containing precipitates having an equivalent circle diameter of 20 nm or more are present per square micrometer of the tempered martensite. Japanese Unexamined Patent Application Publication No. 2010-018862 describes that when a tempered martensite single phase microstructure or a dual phase microstructure composed of ferrite and tempered martensite has an appropriately controlled tempered martensite area ratio and an appropriately controlled distribution of V-containing precipitates precipitating in the tempered martensite, stretch flangeability is improved while ensuring hydrogen embrittlement resistance.
An example of a steel sheet that has both high strength and excellent ductility is a TRIP steel sheet that utilizes transformation induced plasticity (TRIP) of retained austenite. The TRIP steel sheet has a steel sheet microstructure containing retained austenite. When the TRIP steel sheet is worked and deformed at a temperature of the martensite transformation start temperature or more, retained austenite is induced by stress to transform into martensite and large elongation is obtained. However, the TRIP steel sheet has a drawback in that because retained austenite transforms into martensite during blanking, cracks occur at the interface with ferrite and hole expandability is degraded. Thus, high-strength steel sheets having excellent ductility and hole expandability (stretch flangeability) have been developed as disclosed in Japanese Unexamined Patent Application Publication No. 2005-240178 and Japanese Unexamined Patent Application Publication No. 2004-332099.
Japanese Unexamined Patent Application Publication No. 2005-240178 discloses a low-yield-ratio, high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability and achieving a strength TS as high as 980 MPa or higher. That steel sheet has a steel microstructure satisfying the following in terms of area ratio:retained austenite:at least 5%, bainitic ferrite:at least 60%, polygonal ferrite:20% or less (including 0%). Japanese Unexamined Patent Application Publication No. 2004-332099 discloses a high-strength steel sheet having excellent hole expandability and ductility. That steel sheet has a microstructure composed of a total of 34% to 97% of one or both of bainite and bainitic ferrite as a main phase in terms of area ratio, 3% to 30% of austenite as a second phase in terms of area ratio (Vγ), and the balance being ferrite and/or martensite.
In general, DP steel has a low yield ratio since mobile dislocations are introduced into ferrite during martensite transformation, and thus has a low impact energy absorbing property. With regard to the technology described in Japanese Unexamined Patent Application Publication No. 2011-052295, although the stretch flangeability of the steel sheet is enhanced by performing tempering at high temperature for a short time, the elongation is insufficient with respect to the strength of the steel sheet. The technology described in Japanese Unexamined Patent Application Publication No. 2010-018862 also offers insufficient elongation with respect to the strength, and sufficient formability is not obtained. The steel sheet that utilizes retained austenite according to Japanese Unexamined Patent Application Publication No. 2005-240178 has a low impact energy absorbing property due to a low YR of the obtained steel sheet, and thus elongation and stretch flangeability are not enhanced in the high-strength region of 1180 MPa or higher. According to Japanese Unexamined Patent Application Publication No. 2004-332099, elongation is insufficient with respect to the strength of the steel sheet obtained, and sufficient formability is not obtained.
As described above, it is difficult for high-strength steel sheets having a strength of 1180 MPa or higher to achieve an excellent impact energy absorbing property, excellent press formability, elongation, and hole expandability, and excellent delayed fracture resistance. Currently, there is no steel sheet of any type that achieves these properties (yield ratio, strength, elongation, hole expandability, and delayed fracture resistance).
It could therefore be helpful to provide a high-strength cold-rolled steel sheet having excellent elongation, hole expandability, and delayed fracture resistance as well as a high yield ratio, and a method of producing such a high-strength cold-rolled steel sheet.